Prostaglandin endoperoxide H synthases-1 and -2 (PGHS-1 and -2), also known as cyclooxygenases-1 and -2 (COX-1 and COX-2), catalyze the conversion of arachidonic acid (AA) to prostaglandin H2 (PGH2) in the committed step of prostaglandin (PG) biosynthesis. PGHSs are the primary targets of COX inhibitors, which include nonspecific nonsteroidal anti-inflammatory drugs (nsNSAIDs) and COX-2 specific inhibitors called coxibs. COX inhibitors are the most widely used pharmaceutical agents in the U.S. However, the use of these inhibitors carries significant risks. About 20,000 deaths annually are attributable to adverse effects of these drugs, the molecular basis for which is unknown. Using purified human PGHSs, we have discovered that PGHS activities are modulated through an unusual allosteric mechanism by all common fatty acids (FAs) including those that are not PGHS substrates. FAs can stimulate or inhibit PGHS activity with the specific effect being dependent on the PGHS isoform and the FA. The effects occur at physiologic FA concentrations and are observed in cells as well as with purified enzymes. The biochemical basis for the regulation of PGHSs by FAs involves cross-talk between monomers comprising PGHS homodimers. Although the monomers have identical amino acid sequences, the conformations of the two monomers comprising a PGHS homodimer differ. One monomer binds FAs and behaves as an allosteric monomer while the other acts as the catalytic monomer. Finally and importantly, responses of purified human PGHSs to COX inhibitors are modulated by FAs, again depending on the FA, the inhibitor and the PGHS isoform. With different COX inhibitors, FAs can influence binding of an inhibitor to one or to both monomers. The goals of the proposed research are to determine how PGHSs and their responses to widely used COX inhibitors are affected by FAs at the molecular, cellular and whole animal levels. Our underlying hypothesis is that both in vivo PG production and responses to COX inhibitors are significantly modulated by the milieu of FAs in which the enzymes find themselves-the FA tone--and that this FA environment is importantly influenced by the FA composition of the diet. We presume that every individual establishes a FA tone as a consequence of dietary habits in the context of their genetic background. We speculate that certain FA tones predispose susceptible individuals to adverse consequences of COX inhibitors. We expect that our studies delineating FA/COX inhibitor interactions will be a first step leading to changes in the way COX inhibitors are prescribed to people on different diets and to dietary adjustments to provide for the safer use of COX inhibitors.